Photoluminescence wavelength converted light emitting LEDs (“LEDs”) include one or more photoluminescence materials (typically inorganic phosphor materials), which absorb a portion of the excitation light (typically blue) emitted by the LED and re-emit light of a different color (wavelength). Manganese-activated fluoride phosphors such as K2SiF6:Mn4+ (KSF), K2TiF6:Mn4+ (KTF), and K2GeF6:Mn4+ (KGF) have a very narrow red spectrum (Full Width Half Maximum of less than 10 nm for their main emission line spectrum) which makes them highly desirable for attaining high color gamut (NTSC, DCI-P3, Rec2020) in display applications and for attaining a high General Color Rendering Index (CRI Ra) in general lighting applications.
FIG. 1 is a sectional view of a known packaged white light emitting device that utilizes a manganese-activated phosphor material. Referring to FIG. 1, the packaged light emitting device 10 comprises a package 12 having a cavity 14 that contains at least one LED die 16. The cavity 14 is filled with a transparent optical encapsulant having a mixture of a manganese-activated fluoride phosphor and a yellow to green light emitting phosphor such as a garnet-based phosphor material dispersed in the encapsulant.
While manganese-activated fluoride photoluminescence materials are highly desirable for the above reasons, there are several drawbacks that make their widespread use challenging. First, the absorption capability of manganese-activated fluoride phosphors is substantially lower (typically about a tenth) than that of europium-activated red nitride phosphor materials (such as CASN) that are currently commonly used in photoluminescence wavelength converted LEDs. Therefore, depending on the application, in order to achieve the same target color point, the usage amount of manganese-activated fluoride phosphors typically can be from 5 to 20 times greater than the usage amount of a corresponding europium-activated red nitride phosphor. The increased amount of phosphor usage significantly increases the cost of manufacture since manganese-activated fluoride phosphors are significantly more expensive than europium-activated red nitride phosphors (at least five times more expensive). As a result of the higher usage and higher cost, use of manganese-activated fluoride red phosphors can be prohibitively expensive for many applications. Moreover, since a very high photoluminescence material loading in silicone is required to achieve the desired color point this can reduce the stability of the dispensing process making it difficult to reliably dispense in packaged devices.
Another problem with fluoride-based phosphor materials is that they readily react with water or moisture which causes damage to the dopant manganese which leads to a reduction or loss of their photoluminescence emission (i.e. quantum efficiency) of the phosphor. Moreover, the reaction of the fluoride-based compound with water can generate very corrosive hydrofluoric acid that can react with LED packaging material thereby leading to component failure.
A further problem with known constructions especially for “warm white” (i.e. 2500K-3000K Correlated Color Temperature) light emitting devices which use manganese-activated fluoride photoluminescence materials is their poor reliability. Currently, this poor reliability makes it impractical to use light emitting devices that comprise manganese-activated fluoride photoluminescence materials in general light applications.
The present invention intends to address and/or overcome the limitations discussed above by presenting new designs and methods not hitherto contemplated nor possible by known constructions. More particularly, there is a need for a cost-effective light emitting device that utilizes less manganese-activated fluoride photoluminescence material, enables a more stable dispensing process during manufacture, improves reliability—particularly in “warm white” applications, and possesses an optimized LED packaging design that may effectively isolate the fluoride-based photoluminescence material from any water/moisture in the surrounding environment.